The invention relates to a method and equipment for transforming the colours generated by an image display system, in accordance with the limitations which apply in respect of the perception of colours by people who have an abnormal form of colour vision, and the use thereof for adaptation of the colour palette in a manner such that the colours are easily distinguishable from one another for target groups having the relevant form of abnormal colour vision.
The invention relates in particular to a method and equipment for transforming the colours of computer-generated images on an image display system, such as, for example, a cathode ray tube or LCD screen (liquid crystal display).
People who have an abnormal form of colour vision, approximately 8% of the male population and 0.5% of the female population, do not perceive the colours generated by an image display system in the standard manner. As a result certain functions of the image display system cannot be properly utilised by this group of the population. In this context consideration can be given, for example, to the perception of colour-coded information in computer applications, such as control panels for industrial processes and electronically generated geographical and topographical maps.
One aim of the present invention is to provide a method with which developers of computer software and designers of visual information systems are able to perceive the colours they use in a manner which corresponds to the colour perception of a person who has abnormal colour vision. A further aim of the present invention is to provide a method and equipment for transforming a set of colours in such a way that the differences between the colours comply with a pre-set distinguishability criterion, taking account of the ability of the user to distinguish colours, the various features being supported by a computational method by means of which the set of colours concerned can automatically be modified in accordance with the set distinguishability criterion.
To this end the method according to the invention is characterised in that a data entry unit, connected to the image display system, for storing digital colour specifications and system data in a colour memory unit and memory unit is provided, as well as a computing unit, connected to the data entry unit, for transformation of the digital colour specifications of at least one pixel, as a function of the entered colour abnormality data and colour processing commands, comprising the following steps:
a feeding of the digital colour specifications of the colour or set of colours to be transformed and of the colour abnormality and system data required for the transformation into the computing unit,
b calculation, with the aid of the computing unit, of three primary physiological colour signals for an observer with normal colour vision,
c calculation of a second set of three primary physiological colour signals for an observer with abnormal colour vision, as specified by the colour abnormality data,
d calculation of three new digital colour specifications for generation of colours which generate the same primary physiological colour signals for an observer with normal colour vision as the colour signals calculated under c) for an observer with abnormal colour vision,
e calculation of trichromatic components X, Y and Z in the CIE colour specification system which correspond to the new digital colour specifications,
f assessment of the degree of colour difference in pairs of colours within the set of transformed colours, making use of calculations in accordance with colour difference equations which already exist or are still to be developed,
g selecting those colour differences from the colour differences calculated under f) which do not meet a pre-set difference criterion and then modifying the colours concerned, optionally with the assistance of a computational method, such that said colours then comply with the set difference criterion.
With computer-generated colours the luminance levels of the primary colours are set by means of three colour-specific control signals. Each control signal is formed by an analog voltage originating from a digital-to-analog converter (DAC). An 8-bit DAC, with which analog control signals are determined as a function of the digital colour specifications, is frequently used. The digital colour specifications are described by three numerals, which determine the magnitude of the contributions of the three primary colours to the colours to be generated. Assuming the generally used primary colours red (R), green (G) and blue (B), said digital colour specifications are indicated here by numerical values NR, NG, and NB respectively. With an 8-bit DAC these numerals vary from 0 to 255, so that a maximum of 2563 different colours can be generated by combination of the three primary colours of the image display system. Sets of 64 or 256 different colours, which can be made up from a palette of the said 2563 colours, can usually be rendered visible simultaneously by an image display system.
The perception of colours by a person is initiated by absorption of light in three different types of photoreceptors, which are also referred to as the red, green and blue cones. The latter are mainly effective in the long wave, medium wave and short wave regions, respectively, of the visual spectrum, by means of spectral sensitivities l(xcex), m(xcex) and s(xcex) of the photopigments matched to said regions. The primary physiological colour signals L, M and S generated by the cones can be described as the integral of the product of the spectral sensitivities concerned and the radiance of the light generated by the image display system. Said radiance is determined by the digital colour specifications and the spectral distribution of the primary colours concerned plus the so-called gamma functions, which describe the relationship between the relative radiances of the primary colours cR, cG and cB as a function of the respective digital colour specifications NR, NG and NB.
In the case of an abnormal form of colour vision it can be that there are not three but only two types of cones in the retina These so-called dichromats can be subdivided into protanopes, characterised by the lack of red cones (or L receptors), deuteranopes, characterised by the absence of green cones (or M receptors) and tritanopes, characterised by the absence of blue cones (or S receptors). It can also occur that two of the three types of cones have only very slight differences between them as far as their spectral sensitivity is concerned. In the case of the so-called anomalous trichromats, a distinction is made between protanomalopes, characterised by red cones having a spectral sensitivity lxe2x80x2(xcex) which differs very little from that of the green cones, and deuteranomalopes, having a spectral sensitivity mxe2x80x2(xcex) which differs very little from that of the red cones.
As yet little is known about the tritanomalopes, characterised by an abnormal S receptor system. It is possible that in this case there is merely a question of a reduced contribution by the S receptors, which can be described as a relative reduction in the number of S receptors compared with the numbers of L and M receptors. For the time being this assumption also forms the basis for the computational model used in the invention for simulation of persons who have a tritanomaly. This group, that is to say tritanomalopes and tritanopes together, is relatively small; estimates vary from 0.005 to 0.1% of the population.